Graphene represents a planar two-dimensional carbon structure having a sp2 bond, and the graphene has excellent characteristics such as high mechanical features (Young's modulus: 41.8 GPa, tensile strength: 293.3 MPa), high electric conductivity (351 S cm−1), and broad specific surface area (˜2600 m2g−1) and is very stable physically and chemically. Such specific structure and properties of graphene have been studied to be used for various fields such as electronics, sensors, mechanical resonators, energy storage and conversion (super capacitors, batteries, fuel cells, solar cells or the like) and displays.
The graphene is manufactured in various ways, for example by means of mechanical processing, epitaxy, thermal expansion, vapor phase, chemical vapor deposition (CVD), graphene oxidation-reduction, graphite intercalation compound or the like. In order to apply graphene with excellent characteristics to various fields, the graphene should be mass-produced at low temperature. In addition, in order to commercially use graphene products, price competitiveness and stable processes should be ensured.
At the present, an oxidation-reduction method is generally used for manufacturing a single-layered or multi-layered graphene. In the oxidation-reduction method, graphite oxidized to produce a graphite oxide. After that, the prepared graphite oxide is dispersed in a solvent, a graphene layer is separated by means of ultrasonic dispersion or the like to make a thin graphene oxide, and then the graphene oxide is reduced to make graphene. In order to reduce a graphene oxide, high-temperature thermal treatment is performed at 800° C. or above under a hydrogen or argon atmosphere, or a reducing agent such as hydrazine hydrate, sodium borohydrate (NaBH4), sulfuric acid (H2SO4) or the like is used, as well known in the art.
However, such an existing graphene manufacturing method has problems in its economic feasibility and utilization due to high thermal treatment temperature, the use of a reducing agent, low yield caused by bad efficiency, and high production cost. In addition, since impurities are included in the graphene, low purity becomes another problem.
Meanwhile, it has been studied to use graphene with excellent properties as a material for an electrode of a lithium secondary battery. However, the existing graphene study has been limited to an anode material of a secondary battery, and the study has also been focused on a hybrid structure with other nanoparticles. LiCoO2 used as a cathode material of a secondary battery at the present is expensive and may cause safety problems since it is structurally unstable and may generate oxygen. Therefore, there is a demand for developing its alternate material.